For many years, bare boards comprising copper circuitry were finished with eutectic tin-lead solder coating according to the Hot Air Solder Leveling (HASL) process. Due to the Restriction of Hazardous Substances (RoHS) directive, the industry has moved away from using lead as a component of the final finish of bare boards.
Alternative final finishes include organic solderability preservative (OSP), electroless nickel-immersion gold (ENIG), immersion tin, and immersion silver. OSP is an organic coating that is susceptible to chemical and mechanical removal and thus may not adequately protect copper circuitry from oxidation. ENIG is vulnerable to common pollutants and is sensitive to high humidity and tends to fail due to corrosion. Moreover, the process is slow and difficult to control. Finally, the use of gold renders it a relatively expensive process. Immersion tin is susceptible to the formation of copper-tin intermetallic and tin oxide.
A particular problem observed with immersion silver protective coatings is creep corrosion of copper salts at certain bare copper interfaces between copper and silver. Immersion silver may not adequately cover copper surfaces for a variety of reasons. For example, immersion silver processes may not sufficiently coat copper wiring in PCB, particularly at plated through holes and high aspect ratio blind vias. Corrosion at these locations manifests itself as an annular ring surrounding the vias and plated through holes. Some exposed bare copper is present at the edge of soldermask. Additionally, immersion silver is beset by intrinsic pore formation. In other words, immersion silver processes, being self-limiting, deposit relatively thin layers. These thin layers are porous. Finally, silver is susceptible to sulfidation by reduced sulfur compounds (e.g., hydrogen sulfide) present in the environment, particularly at paper processing plants, rubber processing plants, and high pollution environments.
Sufficient sulfidation of silver can result in localized areas of silver sulfide salts that, if they grow large enough, may separate from the silver layer, also forming pores. Exposed areas of copper, which may result from insufficient coverage from the immersion plating process, from intrinsic pores in the layer from the immersion silver process, or from later formed pores caused by sulfidation, are susceptible to creep corrosion. Humidity and environmental pollutants can oxidize and sulfidize the copper, forming copper salts that may creep through any location of insufficient copper coverage by the immersion silver layer.
Immersion silver coatings have been protected with a coating comprising a mercaptan. Mercaptans, however, may not sufficiently protect the board from creep corrosion. Moreover, mercaptan coatings may degrade during assembly processes employing lead-free solders, which typically occur at temperatures above 220° C. and may be as high as 270° C.